1. Technical Field of the Invention
The present invention relates to an implantable cardioverter, especially a defibrillator, with a morphology detector for detection and evaluation of electrocardiogram (EKG) signals for the purpose of discriminating between different tachycardia conditions of the heart.
2. Prior Art
Regarding the background of the invention, it is important to note in principle, that pathological rhythm abnormalities of the heart, so-called xe2x80x9ctachycardias,xe2x80x9d exist as a clinical picture, which can occur either atrial or ventricular, i.e., in the atrium of the heart or in the ventricle. This is a condition of the heart with a pathologically increased heart rate, which can be significantly higher than 100 beats per minute. In this context one generally distinguishes between two different disorder patterns, i.e., the atrial or ventricular fluttering on one hand and the atrial or ventricular fibrillation on the other hand. The xe2x80x9cflutteringxe2x80x9d is a rapid succession of relatively regular heart actions, during which a certain pumping capacity of the heart still remains available. During the xe2x80x9cfibrillationxe2x80x9d on the other hand, an asynchronous action of the heart muscle cells is present, by which an effective pumping action is no longer achieved.
Both of the above conditions can have identical rates, however, they can be vastly different in their stability and, therefore, in their consequences for the patient. The two types of disorder, xe2x80x9cflutteringxe2x80x9d and xe2x80x9cfibrillation,xe2x80x9d therefore, call for different cardiological responses. A modern pacemaker or cardioverter must, therefore, be able to distinguish exactly between conditions of fluttering and conditions of fibrillation, to be able to take the appropriate actions. For this purpose these devices incorporate the above-mentioned morphology detector, which detects and evaluates the EKG signal in a suitable manner. Naturally, a large range of options exists for the actual method by which the signals are processed and translated into action, which are limited primarily by the special location and related conditions of compatibility for a cardioverter or defibrillator.
With respect to the state of the art, it needs to be pointed out that known morphology detectors measure, for instance, the width of the entire depolarization complex of the EKG signal. The measurement of the width of individual spikes does not take place. This is the case, for example, with the detectors according to U.S. Pat. Nos. 5,542,430 and 5,447,519. The morphology detector according to the U.S. Pat. No. 5,560,369 furthermore predominantly addresses an automatic setting of threshold values, but not a discrimination between the tachycardia conditions described above.
With the device for controlling heart arrhythmias presented in the U.S. Pat. No. 5,086,772, it is possible to distinguish, among others, between conditions of a ventricular tachycardia and supraventricular tachycardia. This is done by utilizing the so-called 50% width of the R wave of the EKG signal. A discrimination between xe2x80x9cflutteringxe2x80x9d and xe2x80x9cfibrillationxe2x80x9d does not take place.
From EP 0 592 625 B 1 a cardioverter or defibrillator is known that encompasses an electrogram wave form analyzer. The electrical signals from the patient""s heart are digitized and stored sequentially. These stored signals are scanned within a certain time window and the gain of the signals is practically determined by subtraction. This serves as the basis for delivery of the appropriate therapy by the cardioverter or defibrillator.
The present invention is now based on the object of improving an implantable cardioverter so that its morphology detector can reliably discriminate between different tachycardia conditions of the heart and especially between a xe2x80x9cflutteringxe2x80x9d and a xe2x80x9cfibrillationxe2x80x9d.
This object is met with an implantable cardioverter with a morphology detector having the following characteristics:
a signal width detector (10), by which the width (w) of the EKG signal peak between every two successive crossings (d2) through the isoelectric line (7) of the EKG can be determined,
a threshold value discriminator (11) to determine whether the EKG signal between two such successive crossings (d1, d2) exceeds a defined threshold value (T, a1), and
a comparator (12) for the signal width (ws), which is coupled to the signal width detector (10) and the threshold value discriminator (11), to compare the current value of the signal width (ws) selected by the threshold value discriminator (11) and determined by the signal width detector (10) to a pre-set selection parameter in order to discriminate between two different conditions of tachycardia.
The present invention makes use of the discovery that during a fluttering condition of the heart, the EKG signal returns to the baseline or isoelectrical zero line and displays small peaks (spikes). During a fibrillation condition of the heart, by contrast, the EKG signal on the average deviates more strongly from the baseline and has significantly wider peaks. This will be explained in more detail later, in connection with the description of the figures.
The morphology detector proposed in accordance with the invention now utilizes the width of the EKG signal between two zero crossings through the baseline in such a way that only those EGK signal peaks are monitored whose amplitude exceeds a defined threshold value. To provide the above functionalities, the above-described signal width detector and the threshold value discriminator act together. With respect to the term xe2x80x9cdefined threshold valuexe2x80x9d it should be noted that this does not necessarily have to be a constant threshold value. It may also be a so-called xe2x80x9cadaptive threshold valuexe2x80x9d, which is determined dynamically based on a continual evaluation of the EKG signal on the basis of the respective peak amplitude. An adaptive threshold setting of this type, incidentally, is known in the field of cardioverters and is being used in practice.
The comparator for the signal width that is furthermore provided compares the current value of the signal width to a pre-set selection parameter, which separates the tachycardia conditions of xe2x80x9cflutteringxe2x80x9d and xe2x80x9cfibrillationxe2x80x9d as reliably as possible. A comparison is, therefore, made whether the determined value of the signal width, which may optionally be processed by means of customary statistical methods and evaluations, exceeds the selection parameter, thus indicating the likelihood that a fibrillation condition (wide peaks) is present, or whether the determined signal width value is lower than the selection parameter, thus indicating a fluttering of the heart, based on the accompanying smaller peaks.